It could be argued that a life free of lies and sarcasm would be a happier one. But the inability to recognize others’ insincerity can also be hazardous — and a warning sign of a form of dementia.
Researchers have known for years that people with frontotemporal dementia — a type of dementia that affects the frontal and temporal lobes of the brain — often lose the ability to detect sarcasm and lies, while people who age normally don’t experience this. (More on Time.com: Photos: My Aging Father’s Decline: A Son’s Photo Journal)
Because frontotemporal dementia affects regions of the brain involved in complex, higher-order processes like personality and behavior, patients may show certain striking symptoms like behaving in compulsive, socially inappropriate ways or suddenly changing religions. They also become gullible, as evidenced by the many patients who lose money to hucksters and scammers online.
What a team of researchers from the University of California, San Francisco, wanted to find out was the strength of the association between frontotemporal dementia and the failure to identify insincerity or deception. Would these deficits show up in patients whose brains scans suggested the mental disorder? And would the link be strong enough to identify those at risk?
The researchers asked 175 older adults, more than half of whom had some form of dementia, to watch a video of two people speaking. One of the speakers occasionally lied or used sarcasm — made clear by verbal and non-verbal cues. Viewers were then asked a series of yes or no questions about what they saw. Researchers also took MRI scans of the participants’ brains. (More on TIME.com: Brain Exercises Delay, But Can’t Prevent, Dementia)
Mentally healthy older participants had no problem identifying the speaker’s lies and sarcasm. But those whose brain scans showed signs of frontotemporal dementia were blind to the deception. People with other types of dementia, including Alzheimer’s, performed somewhat better on the lie-detecting task.
The study’s authors concluded that the inability to perceive lies or sarcasm may help doctors identify patients with dementia, especially since other early behavioral warning signs tend to be overlooked, misdiagnosed as symptoms of depression, mid-life crisis or Alzheimer’s. (More on TIME.com: Alzheimer’s: Largely a Woman’s Issue)
“We have to find these people early,” said senior author and UCSF neuropsychologist Katherine Rankin in a press release, noting that early detection offers the best chance for intervention and treatment.
Frontotemporal dementia is rare, making up perhaps 5% of patients with dementia; by comparison, Alzheimer’s disease accounts for nearly 80% of such patients. Frontotemporal dementia tends to occur at a younger age than Alzheimer’s, typically between the ages of 40 and 70. (More on TIME.com: Cover Story: Alzheimer’s Unlocked)
“If somebody has strange behavior and they stop understanding things like sarcasm and lies, they should see a specialist who can make sure this is not the start of one of these diseases,” said Rankin.
The findings were presented at the annual meeting of the American Academy of Neurology in Hawaii.